linux-kernel - Re: [PATCH 2/2] selftests/fpu: Add an FPU selftest

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Date:   Fri, 19 Jun 2020 11:00:28 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     X86 ML <x86@...nel.org>, jpa@...nelbug.mail.kapsi.fi,
        Dave Hansen <dave.hansen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] selftests/fpu: Add an FPU selftest

On Fri, Jun 19, 2020 at 10:41 AM Borislav Petkov <bp@...en8.de> wrote:
>
> From: Petteri Aimonen <jpa@....mail.kapsi.fi>
>
> Add a selftest for the usage of FPU code in kernel mode.
>
> Currently only implemented for x86. In the future, kernel FPU testing
> could be unified between the different architectures supporting it.

Acked-by: Andy Lutomirski <luto@...nel.org>

except:

> +#
> +# CFLAGS for compiling floating point code inside the kernel. x86/Makefile turns
> +# off the generation of FPU/SSE* instructions for kernel proper but FPU_FLAGS
> +# get appended last to CFLAGS and thus override those previous compiler options.
> +#
> +FPU_CFLAGS += -mhard-float -msse
> +ifdef CONFIG_CC_IS_GCC
> +  ifeq ($(call cc-ifversion, -lt, 0701, y), y)
> +    # Stack alignment mismatch, proceed with caution.
> +    # GCC < 7.1 cannot compile code using `double` and -mpreferred-stack-boundary=3
> +    # (8B stack alignment).
> +    FPU_CFLAGS += -mpreferred-stack-boundary=4
> +  else
> +    FPU_CFLAGS += -msse2
> +  endif
> +endif

This should be cc-option, not cc-ifversion, I think.  But maybe we
should consider dropping the problematic GCC version instead?  The old
GCC versions with stack alignment problems are seriously problematic
for x86 kernels, and I don't really trust kernels built with them.